The present invention generally relates to a display apparatus. More specifically, the invention is concerned with a display apparatus for a television receiver set, a computer terminal or the like in which a cathode ray tube is employed.
In the conventional television receiver sets, scanning operation is performed with such magnitude or size of deflection (hereinafter referred to as the deflection size) that a screen of a cathode ray tube in which images, pictures, etc. are displayed is generated in a greater size than that of the faceplate of the cathode ray tube (known as the over-scan operation). Consequently, variation in the screen size brought about by variation in luminance (which may also be referred to as brightness) is relatively inconspicuous. In recent years, as the personal computer becomes more popular, there arises a demand for the capability of displaying pictures, images and so forth generated by a personal computer on a television receiver set.
In order to display the image information generated by the personal computer without any shortage, it is required to perform the scanning operation with such deflection size that the screen size or display area is smaller than that of the faceplate of the cathode ray tube (known as the under-scan operation). In that case, edges of the display area appear on the faceplate. Consequently, variation of the deflection size taking place in accompanying change of luminance or brightness becomes noticeable, giving rise to a problem.
On the other hand, in the conventional display apparatus for the personal computer, the under-scan operation is adopted. However, because the brightness or luminance of the screen is not so high as in the case of the television receiver set, variation of the screen size or display area can be neglected. Recently, however, in an effort to ensure enhanced picture quality for the display of motion pictures, there is a tendency of increasing the brightness or luminance by increasing the beam current of the cathode ray tube. As a result, the display apparatus for the computer terminal also suffers the problem that relatively conspicuous variation of the deflection size (in other words, noticeable variation of the screen size or display area) accompanies a change of luminance.
Under the circumstances, there has already been developed a technique for suppressing variation of the deflection size brought about as luminance changes, as is disclosed in Japanese Utility Model Application Publication No. 20572/1991 (JP-UA-3-20572). According to this conventional technique, variation of the beam current occurring with the change in luminance is detected, wherein the horizontal deflection size is so controlled on the basis of the detected beam current that the deflection size can remain substantially invariable. With this conventional method, variation of the horizontal deflection size brought about by the change of luminance can certainly be suppressed at least partially. However, it is impossible to exclude completely variation of the horizontal deflection size, due possibly to by the fact that the change of the beam current of the cathode ray tube can not always provide a good basis for the availability of an optimal horizontal deflection size correcting quantity. In general, when an anode voltage of the cathode ray tube changes, the screen size or display area on the faceplate (i.e., deflection size) changes substantially in inverse proportion to a square root of the anode voltage. Further, when output impedance of a flyback transformer which supplies the anode voltage for the cathode ray tube is regarded as being constant, the anode voltage changes substantially in accordance with a linear function as the beam current changes. On the other hand, the amplitude of the horizontal deflecting current exhibits an essentially linear relationship with the horizontal deflection size. Accordingly, variation of the horizontal deflection size can not satisfactorily be eliminated even when the horizontal deflecting current is changed as a linear function of the change of the beam current.
FIG. 7 shows a circuit disclosed in the Japanese publication mentioned previously (see FIG. 4 of the same). Referring to FIG. 7, such circuit arrangement is adopted that a horizontal size correcting signal is generated substantially as a linear function of the change in the beam current, wherein a horizontal deflecting current is changed on the basis of the horizontal size correcting signal. Accordingly, with the prior art technique disclosed in the above-cited publication, variation of the horizontal deflection size which accompanies the change of luminance can be suppressed to a certain extent. However, it is impossible to avoid completely the variation of the horizontal deflection size.